Generally, when a synchronous rotary machine, e.g., a synchronous motor of the permanent-magnet field type is used as an AC servo motor, the outer shell of the motor has formed thereon connection portions, i.e., close-fitting engagement portions or mounting surfaces to enable precise attachment to the motor mount of the machine tool or the like. Hight accuracy is required in the concentricity of the connection portions of the outer shell of the motor to the rotor shaft, in the squareness of the mounting surfaces of the outer shell of the motor to the axis of the rotor shaft, etc.
Use has been made in the past of a synchronous rotary machine in which the stacked iron core of the stator forms a part of the outer shell of the rotary machine. The construction of a conventional, general stator assembly of a synchronous rotary machine is shown in FIG. 1. Referring to the figure, a stator iron core 2 is made of a number of thin plates stacked in the axial direction. The stator iron core 2, having thereon a winding, is provided at opposite ends thereof with thick end plates 3 and 4. As also shown in FIG. 2, a plurality of tie rods 5 are fitted into fitting grooves formed both in the stator iron core 2 and the end paltes 3 and 4. The tie rods 5 are welded to the stator iron core 2 and the end plates 3 and 4. From this construction, the thin plates of the stator iron core 2 are fixed to each other, and the end plates 3 and 4 are firmly fixed with respect to the stator core 2.
After the welding is finished, the end portions of the end plates 3 and 4 are machined to a precision finish to form connection portion 3a and 4a. Meanwhile, brackets 6 and 7 are machined to a precision finish with holes 6a and 7a to fit the connection portions 3a and 4a of the end plates 3 and 4 and with mounting holes 6b and 7b for the bearing members. Further, the fore bracket 6 is machined to a precision finish with a connection portion 6c and a mounting surface 6d to be fit to the mount of the machine tool. After the machining work is finished, the brackets 6 and 7 are fitted into the connection portions 3a and 4a of the end plates 3 and 4, respectively, and then firmly fixed by means of a plurality of tie bolts 8.
In such a conventional stator, since the welded assembly of the stator core 2 and the end plates 3 and 4 is assembled to the brackets 6 and 7 after the welded assembly and brackets 6 and 7 are machined individually as described above, the number of portions to be precisely machined increases. Further, the individual dimentional errors produced by the machining are accumulated by the joining of the welded assembly and the brackets, so the final accuracy of the stator is decreased. Furthermore, since the stator iron core 2 is welded, a large strain is caused by the welding in the stator iron core 2, as shown by the two-dot chain line in FIG. 2, inviting an increase in the fluctuations in the output-torque of the synchronous rotary machine.
FIG. 3 shows the construction of another conventional stator. Referring to this figure a fore bracket 6 and a rear bracket 7 are directly attached to the opposite ends of the stacked stator iron core 2 with the winding. The stator iron core 2 and the brackets 6 and 7 are tightened and fixed by means of a plurality of tie bolts 8. Mounting holes 6b and 7b of the brackets 6 and 7 for the bearing members, the connection portion 6c and the mounting surface 6d of the fore bracket 6 are formed by machining after the stator iron core and the brackets are fixed in place by the bolts 8. Therefore, the problem of accumulation of the dimentional errors does not occur. Further, it is not necessary to firmly form the stacked stator iron core 2 before the stator iron core 2 and the brackets 6 and 7 are assembled. It is enough to perform some simple treatment, e.g., TIG-welding, to prevent the core from falling out of shape. Therefore, the stator iron core 2 will not deform much under heat.
However, in the case of the latter method, the stacked stator iron core 2 will deform when subjected to a large external force. Therefore, when tightening the tie bolts 8, as schematically shown in FIG. 4, variations in the amount of tightening of the tie bolts 8 will result in a large deviations in the concentricity between the brackets 6 and 7 with respect to the stator iron core 2. This makes subsequent machining difficult. Further, since the stator iron core 2 can stretch and deform in the axial direction after tightening, it is difficult to keep the shape of the stator regular. Thus, subsequent machining becomes difficult, making it impossible to obtain high machine accuracy, or sufficient strength.